This invention relates to a semiconductor sensor and, more particularly, to a semiconductor sensor having a fault detection circuit.
FIGS. 6 and 7 illustrate equivalent circuits of a conventional semiconductor sensor to which the present invention pertains. In FIGS. 6 and 7, the semiconductor sensor comprises a semiconductor sensor circuit 3 in which a semiconductor sensor element 1 which detects and measures a physical parameter such as pressure, velocity, acceleration or the like, and a semiconductor differential amplifier circuit 2 which processes an output signal from the semiconductor sensor element 1 are integrated. The semiconductor sensor circuit 3 is contained within a single case as shown by dot-and-dash line as a semiconductor sensor apparatus 4, which is connected to a power source 5, an output 6 and ground 7. The semiconductor sensor circuit 3 comprises a power source terminal 5a, an output terminal 6a and a ground terminal 7a, and the semiconductor sensor apparatus 4 comprises a power source terminal 5b, an output terminal 6b and a ground terminal 7b. The output terminal 6b is connected to an analogue-digital converter (A-D converter) 9 of a control unit 8 to which the signal from the semiconductor sensor apparatus 4 is supplied, the input impedance of the A-D converter 9 being designated by reference numeral 10.
FIG. 7 illustrates an equivalent circuit of an output portion of the semiconductor sensor circuit 3 illustrated in FIG. 6. As apparent from this equivalent circuit, the semiconductor sensor circuit 3 comprises an emitter-grounded NPN transistor 11 which is the output of the semiconductor sensor circuit 3, a parasitic diode 12 of which cathode is connected to the collector of the NPN transistor 11 and the anode is connected to the ground, a first PNP transistor 13 which is the load of the NPN transistor 11 and a second PNP transistor 14 constituting a current mirror circuit together with the first PNP transistor 13. The second PNP transistor 14 is connected as its base to the base of the first PNP transistor 13 and at its emitter to the emitter of the first PNP transistor 13 and its collector is connected to its own base. A constant current source 15 is connected to the base and the collector of the second PNP transistor to determine the constant current flowing through the first PNP transistor 13, and the base of the NPN transistor 11 is connected to an amplifier circuit 16. The internal impedance of the semiconductor sensor circuit 3 between the power source 5 and the ground 7 is designated by reference numeral 17.
The operation of the conventional semiconductor sensor having the above-described structure will now be described. In FIG. 6, the output from the semiconductor sensor element 1 of an acceleration sensor, for example, is amplified by the differential amplifier circuit 2 and input into the A-D converter 9 of the control unit 8 through the output terminals 6a and 6b. The A-D converter 9 converts the analogue signal into the digital signal and is used in the control by the control unit 8.
The maximum value of the output voltage V.sub.0(max) of the semiconductor sensor circuit is expressed by EQU V.sub.0(max) =V.sub.cc -V.sub.sat13 ( 1)
where V.sub.cc is voltage at the power source terminal 5a, and V.sub.sat13 is the saturation voltage of the first PNP transistor 13.
Also, the minimum value of the output voltage of the semiconductor sensor circuit 3 is expressed by EQU V.sub.0(min) =V.sub.sat11 ( 2)
where V.sub.sat11 is the saturation voltage of the NPN transistor 11.
Thus, the output voltage from the semiconductor sensor apparatus 4 falls within the range of from V.sub.sat11 to V.sub.cc -V.sub.sat13.
A description will now be given in terms of the output voltage produced when a breakage takes place between the power source terminals 5a and 5b. In this case, since the semiconductor sensor circuit 3 is provided with no electrical power, the NPN transistor 1 and the first PNP transistor 13 are interrupted. Therefore, since the output terminal 6b rises to a very high impedance, the voltage is substantially equal to the ground potential due to the input impedance 10 of the A-D converter 9.
When a breakage takes places between the ground terminals 7a and 7b, while the NPN transistor 11 and the PNP transistor 13 are similarly interrupted, the electrical current flows from the power source terminal 5a to the input impedance 10 of the A-D converter 9 through the semiconductor sensor element 1 and the internal impedance 17 and the parasitic diode 12. The output voltage V.sub.0(7open) from the semiconductor sensor apparatus 4 at this time is: EQU V.sub.o(7open) =(V.sub.cc -V.sub.F12).times.[R.sub.I(A-D) ]/[R.sub.I(A-D) +R.sub.G //R.sub.i ( 3)
where, R.sub.G is a gauge resistance of the semiconductor sensor element 1 (i.e., impedance between the power source 5 and the ground 7), R.sub.i is a resistance of the internal impedance, R.sub.I(A-D) is the input impedance 10 of the A-D converter 9, and V.sub.F12 is a forward voltage of the parasitic diode 12. Therefore, when V.sub.cc =5 V, R.sub.G =3 k.OMEGA., R.sub.i =10 k.OMEGA., R.sub.I(A-D) =10 M.OMEGA. and V.sub.F12 =0.6 V, then V.sub.0(7open) equals to about 4.40 V.
As has been described, when the connection breakage takes place in the power source terminals 5a and 5b and the output terminals 6a and 6b, the outputs are at about the ground potential, and when the connection breakage takes place at the ground terminals 7a and 7b, the output voltage is about 4.4 V when V.sub.cc =5 V. However, when the disconnection takes place either at the power source terminals 5a and 5b and the output terminals 6a and 6b,the grounding is achieved through the input impedance 10 of the A-D converter 9, resulting in a high impedance relative to the ground. The output voltage is subjected to noise on the wiring conductor extending from the output terminal 6b to the control unit 8, a very unstable potential which may often fall within the normal output voltage range during the normal semiconductor sensor operation. Therefore, it is often impossible to distinguish the output signals generated during the normal semiconductor sensor operation from the output signals generated when the electrical connection is broken and disconnected. Also, when the ground terminals 7a and 7b are disconnected, the potential at the output terminal 6b is at about 4.4 V, and if it is assumed that V.sub.sat13 =0.2 V as in the above equation (1), the maximum output voltage V.sub.0(max) is 4.8 V. Thus, the output voltage can be equal to the potential of the output voltage generated when the ground terminals 7a and 7b are disconnected even when the semiconductor sensor apparatus 4 is correctly operating, making it impossible to distinguish the output signals generated when the connection is broken from those generated during normal operation.
Thus, in either case, it is impossible to detect the breakage of the power source terminals 5a and 5b, the output terminals 6a and 6b, and the ground terminals 7a and 7b of the semiconductor sensor apparatus 4 on the basis of the output voltage from the output terminals 6b.